Critical CVSS 8.1 Actively exploited
CVE-2021-27137
An issue was discovered in router/upnp/src/ssdp.c in DD-WRT before 45724. An unsafe strcpy in the UPnP handling functionality allows an unauthenticated remote attacker to send a request that would overflow an internal fixed buffer. Exploitation requires the DD-WRT user to enable UPnP (which is off by default, and only listens on internal interfaces by default). This occurs in ssdp_msearch (reachable by an M-SEARCH request).
What this means
- Exposure
- Exploitable remotely over the network, without authentication and with no action from the victim. The attack does require particular conditions, which makes it less systematic.
- Impact
- An attacker can read sensitive data, modify or destroy data and take the service offline.
- Weakness
- A stack overflow overwrites the return address, which often leads to code execution.
- Likelihood
- Exploitation is not hypothetical: CISA has observed it in the wild.
What to doTop priority: CISA sets the remediation deadline at 24 July 2026. Start with the instances exposed to the internet.
Read automatically from the CVSS vector, the weakness type (CWE) and the EPSS score. The technical description above remains the one published by NIST.